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use strict;
use warnings;
...
sub pretty {
    return shift =~ s/\d \K (?= (?: \d{3} )+ \b )/_/xgr;
}

start: 0, end: 9, parts: 10
   0 => 0
...
   9 => 1_124_156_267.35
  10 => 1_124_246_357.5